Amorphous thermoplastic resins such as styrene-based resin, acrylic resin and polycarbonate resin and curable resins such as unsaturated polyester resin and diallyl phthalate resin have good transparency to light of wavelengths in the visible region. These resin materials are general-purpose transparent resin materials having low specific gravity and an excellent characteristic well-balanced in terms of mechanical characteristics such as low-temperature moldability, mass-productivity, toughness, flexibility, and shock resistance when compared with inorganic glass materials. However, refractive indices of these transparent resin materials are determined by their constituents, so that the transparent resin materials have a narrow control range of optical characteristics compared with conventional optical glass materials. As one of means for increasing a refractive index of the transparent resin material, an attempt to incorporate, into a resin material, an element having a large atomic number such as bromine or sulfur which is a component contributing to an increase in dielectric constant has been made but there remains a problem that a variable range of the refractive index is still narrow.
In order to remedy such a problem of organic polymeric materials, incorporation of fine particles of inorganic metal oxide, inorganic metal sulfide, or the like into a resin matrix so as to utilize their high refractive indices has been attempted.
For example, Japanese Laid-Open Patent Application (JP-A) Hei 1-306477 has disclosed a hard coating agent, for an optical material, comprising an oxide sol such as an antimony oxide sol and siloxane as a matrix component. In this case, a silane coupling agent or siloxane contributes to dispersion of the colloid. However, in order to apply this coating agent to a bulk material such as a lens, a decrease in particle size of the fine oxide particles in the resin matrix and an improvement in dispersibility of the fine oxide particles remain to be solved as big problems.
In order to improve the dispersibility of the oxide fine particles in the matrix, studies on introduction of a dispersant other than siloxane or a functional group having a dispersing function into the matrix have been made.
JP-A Hei 5-25320 has disclosed a curable composition comprising a thermosetting resin such as acrylic resin or unsaturated polyester resin, an inorganic filler of fine powdery titanium oxide or the like, and a dispersant consisting of a phosphate compound having a terminal aryl group. JP-A 2002-55225 has disclosed an active energy-polymerizable resin layer, as a hard coating surface layer of an optical filter, containing inorganic particles treated with an organic compound having an active energy-curable group and an acidic group such as phosphoric group, sulfonic group or carboxylic group. JP-A (Tokuhyo) 2004-524396 has disclosed a composite composition, as an electric or optic device, comprising inorganic particles and a polymer having a side-chain containing oxysilane group, phosphonate group, sulfide group, amino group, or sulfonate group. JP-A 2002-105325 has disclosed a composition prepared by dispersing ultrafine semiconductor particles in a resin matrix having a polymer chain copolymerized with radical-polymerizable phosphine oxide as a ligand for the fine particles. JP-A 2002-20678 has disclosed a dispersing method of fine metal oxide particles for a coating film, in which an adsorptive segment is imparted to a polymer main chain. In these dispersion methods, interaction between a surface of inorganic fine particle and a functional group such as phosphoric group, phosphine oxide or carboxyl group is ensured but there arises such a problem that a resultant bonding strength is still weak. Therefore, particularly, with respect to particles of a particle size of 10 nm or less, a satisfactory dispersion performance has not yet obtained, so that the use of the resultant dispersoid is limited to a thin coating film. In the case of requiring thermal moldability, it is required that dispersibility of fine nano-oxide particles is not destroyed by flow of a thermoplastic matrix, which makes the surface treating technique of the fine oxide particles further difficult.
The problem of the low dispersibility of the fine nano-oxide particles is also involved, in addition to the transparent optical material, in resin materials required to increase their strength by dispersing fine oxide particles; shielding materials for radiation, ultraviolet rays, visible rays, infrared rays, and the like on the basis of an optical characteristic of fine metal oxide particles; nonlinear materials based on plasmon in fine particles; and the like.
As described above, the dispersing technique for fine oxide particles, particularly those of nanometer size has been required.